Caltech's Engineering and Science magazine

LIGO

Above, LIGO SURF students, class of 2015, in the control room at the LIGO facility in Livingston, Louisiana. The large monitor over their heads displays incoming data in real time. Credit: LIGO/LLO/Caltech

Over the following summer, 27 students in Caltech’s Summer Undergraduate Research Fellowships (SURF) program got an even more intimate look at LIGO: they became full partners in one of the biggest, most complex physics experiments ever. Their contributions ranged from creating hardware and software for LIGO’s current upgrade to helping design the next generation of upgrades.

Thanks to SURF, students have been part of LIGO since the 1990s—more than 350 of them. Some have gone on to careers with LIGO, and some are now mentoring students themselves.

Caltech president Thomas Rosenbaum inspects a vacuum chamber at the Laser Interferometer Gravitational-wave Observatory (LIGO) in Hanford, Washington, during a tour lead by observatory head Frederick Raab (right) at the May 19 Advanced LIGO dedication. Inside the chamber, in an ultra-high vacuum environment, several pristine mirrors hang in carefully balanced suspension, directing laser light into the gravitational-wave detector’s 4-kilometer beam paths. LIGO was designed and is operated by Caltech and MIT, with funding from the National Science Foundation (NSF). Advanced LIGO, also funded by the NSF, is expected to begin its first searches for gravitational waves this fall, possibly as you are reading these pages.

The Advanced LIGO Project is a major upgrade that should increase the sensitivity of the detector by a factor of 10 and provide a 1,000-fold increase in the number of astrophysical candidates for gravitational-wave signals. “Advanced LIGO represents a critically important step forward in our continuing effort to understand the extraordinary mysteries of our universe,” said NSF director France Córdova (PhD ’79) at the dedication. “It gives scientists a highly sophisticated instrument for detecting gravitational waves, which we believe carry with them information about their dynamic origins and about the nature of gravity that cannot be obtained by conventional astronomical tools.”

“We listen for changes in the separation of mirrors over the 4-kilometer length of each laser-interferometric detector. But thermal energy in the 0.4 millimeter diameter glass strings that hold up 40-kilogram mirrors also causes ringing sounds that we call ‘violin modes.’ And a hiss comes from the quantum nature of the light: fluctuations in the nothingness of empty space that interfere with our pure laser beam.”

Rana Adhikari, professor of physics, talks about noises from the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors. The aim of LIGO is to measure the stretching and squeezing of space-time. Scientists listen to the detector outputs—which are sometimes disturbed by things from the earth, such as earthquakes or traffic—using headphones.

In the Winter 2015 issue of E&S, we examined some of the visualizations created by the most sensitive detectors in the world—those associated with the Laser Interferometer Gravitational-Wave Observatory, or LIGO. Here we take a look at the next step in the LIGO story.

The Advanced LIGO Project, a major upgrade that will increase the sensitivity of the Laser Interferometer Gravitational-wave Observatories instruments by a factor of 10 and provide a 1,000-fold increase in the number of astrophysical candidates for gravitational wave signals, was officially dedicated May 19 in a ceremony held at the LIGO Hanford facility in Richland, Washington.

LIGO was designed and is operated by Caltech and MIT, with funding from the National Science Foundation (NSF). Advanced LIGO, also funded by the NSF, will begin its first searches for gravitational waves in the fall of this year.

Researchers installing some of the small suspended LIGO mirrors in the vacuum system.Credit: Courtesy of LIGO Laboratory

The dedication ceremony featured remarks from Caltech president Thomas F. Rosenbaum, the Sonja and William Davidow Presidential Chair and professor of physics; Professor of Physics Tom Soifer (BS ’68), the Kent and Joyce Kresa Leadership Chair of Caltech’s Division of Physics, Mathematics and Astronomy; and NSF director France Córdova (PhD ’79).

“We’ve spent the past seven years putting together the most sensitive gravitational-wave detector ever built. Commissioning the detectors has gone extremely well thus far, and we are looking forward to our first science run with Advanced LIGO beginning later in 2015. This is a very exciting time for the field,” says Caltech’s David H. Reitze, executive director of the LIGO Project.

“Advanced LIGO represents a critically important step forward in our continuing effort to understand the extraordinary mysteries of our universe,” says Córdova. “It gives scientists a highly sophisticated instrument for detecting gravitational waves, which we believe carry with them information about their dynamic origins and about the nature of gravity that cannot be obtained by conventional astronomical tools.”

“This is a particularly thrilling event, marking the opening of a new window on the universe, one that will allow us to see the final cataclysmic moments in the lives of stars that would otherwise be invisible to us,” says Soifer.

Predicted by Albert Einstein in 1916 as a consequence of his general theory of relativity, gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe—for example, by the collision of two black holes or by the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much in the same way as radio waves are produced by accelerating charges, such as electrons in antennas. As they travel to Earth, these ripples in the space-time fabric bring with them information about their violent origins and about the nature of gravity that cannot be obtained by other astronomical tools.

Although they have not yet been detected directly, the influence of gravitational waves on a binary pulsar system (two neutron stars orbiting each other) has been measured accurately and is in excellent agreement with the predictions. Scientists therefore have great confidence that gravitational waves exist. But a direct detection will confirm Einstein’s vision of the waves and allow a fascinating new window into cataclysms in the cosmos.

LIGO was originally proposed as a means of detecting these gravitational waves. Each of the 4-km-long L-shaped LIGO interferometers (one each at LIGO Hanford and at the LIGO observatory in Livingston, Louisiana) use a laser split into two beams that travel back and forth down long arms (which are beam tubes from which the air has been evacuated). The beams are used to monitor the distance between precisely configured mirrors. According to Einstein’s theory, the relative distance between the mirrors will change very slightly when a gravitational wave passes by.

The original configuration of LIGO was sensitive enough to detect a change in the lengths of the 4-km arms by a distance one-thousandth the size of a proton; this is like accurately measuring the distance from Earth to the nearest star—3 light years—to within the width of a human hair. Advanced LIGO, which will utilize the infrastructure of LIGO, will be 10 times more sensitive.

Included in the upgrade were changes in the lasers (180-watt highly stabilized systems), optics (40-kg fused-silica “test mass” mirrors suspended by fused-silica fibers), seismic isolation systems (using inertial sensing and feedback), and in how the microscopic motion (less than one billionth of one billionth of a meter) of the test masses is detected.

The change of more than a factor of 10 in sensitivity also comes with a significant increase in the sensitive frequency range. This will allow Advanced LIGO to look at the last minutes of the life of pairs of massive black holes as they spiral closer, coalesce into one larger black hole, and then vibrate much like two soap bubbles becoming one. It will also allow the instrument to pinpoint periodic signals from the many known pulsars that radiate in the range from 500 to 1,000 Hertz (frequencies that correspond to high notes on an organ).

Advanced LIGO will also be used to search for the gravitational cosmic background—allowing tests of theories about the development of the universe only 10-35 seconds after the Big Bang.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of some 950 scientists at universities around the United States and in 15 other countries. The LSC network includes the LIGO interferometers and the GEO600 interferometer, located near Hannover, Germany, and theand and the LSC works jointly with the Virgo Collaboration—which designed and constructed the 3-km-long Virgo interferometer located in Cascina, Italy—to analyze data from the LIGO, GEO, and Virgo interferometers.

Several international partners including the Max Planck Institute for Gravitational Physics, the Albert Einstein Institute, the Laser Zentrum Hannover, and the Leibniz Universität Hannover in Germany; an Australian consortium of universities, led by the Australian National University and the University of Adelaide, and supported by the Australian Research Council; partners in the United Kingdom funded by the Science and Technology Facilities Council; and the University of Florida and Columbia University, provided significant contributions of equipment, labor, and expertise.

In their search for gravitational waves—stretches in space-time produced by dramatically violent events in the distant universe—researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) have created some of the most sensitive detectors in the world. Unfortunately, these detectors also pick up on lots of other disturbances—for example, strong winds or the sounds of a passing truck. Students from the Data Visualization Summer Internship program—operated by faculty members from Caltech, JPL, and Art Center College of Design—were tasked with determining if a blip on the detector is a gravitational wave or instead just a signal from ordinary bumps and shakes. To do this, they developed these colorful visualizations to represent how signals are related to known events. The dial on the left marks the time at which each event was recorded; on the right, the height of the bars indicates the strength of each event while the lines connecting the bars indicate how strongly these events are correlated to one another. Coupled with the existing analytical methods used by LIGO, this new way of looking at things will help researchers identify and eliminate terrestrial noise sources, directly improving the astrophysical reach of the LIGO detectors—and making those elusive gravitational waves just that much easier to detect.